In the LTE-Advanced developed from the 3rd Generation Partnership Project Long Term Evolution (3GPP LTE), it is under consideration to introduce uplink coordinated multiple point transmission and reception (UL CoMP). The CoMP refers to a technique of improving the throughput of a terminal in a cell edge region such that a plurality of cells (base stations) perform transmission and reception of a signal with a radio communication terminal apparatus (hereinafter, referred to as “terminal”) in a coordinated manner. The UL CoMP improves the reception quality by receiving a signal transmitted from one terminal at a plurality of cells (base stations) and synthesizing the received signals. Further, in order to reduce influence of intercell interference, a plurality of cells in a CoMP set (a group of cells that perform transmission and reception in a coordinated manner) schedule the terminal in a coordinated manner not to increase intercell interference.
In Non-Patent Literature 1, in order to further increase the performance improvement effect of the UL CoMP, it has been discussed to introduce a multiple user-multiple input multiple output (MU-MIMO) technique between a terminal to which the UL CoMP is applied, that is, a terminal of which transmission signal is received and synthesized at a plurality of cells (hereinafter, referred to as “CoMP terminal”) and a terminal to which the UL CoMP is not applied (hereinafter, referred to as “non-CoMP terminal”). The MIMO is a technique of simultaneously spatially multiplexing and transmitting different signal sequences at the same frequency by the use of a plurality of antennas at both transmitter and receiver stations. Further, the MU-MIMO can improve frequency use efficiency of a system through a technique of performing MIMO communication between a plurality of terminals and a base station.
In the MU-MIMO communication, in order to demultiplex signals of different terminals, it is necessary to transmit an orthogonal data demodulation reference signal (hereinafter, referred to as “DM-RS”) between terminals. In the uplink of the conventional LTE in which the CoMP is not performed, the MU-MIMO technique is employed between the non-CoMP terminals, and an orthogonal cyclic shifted Zadoff-Chu (CS-ZC) sequence is used between terminals as the DM-RS. The CS-ZC sequence is a sequence obtained by cyclic-shifting a ZC sequence, and a plurality of CS-ZC sequences obtained by cyclic-shifting the ZC sequence of the same sequence number can be made orthogonal to one another by setting a value larger than a maximum propagation delay time of a transmission signal of a terminal as a cyclic shift amount. Further, in the uplink of the LTE, a sequence group in which a ZC sequence number for the DM-RS of each transmission bandwidth available within a cell is defined is configured, and one sequence group is assigned to each cell. By informing terminals within a cell of a sequence group number, the base station may not perform signaling of the ZC sequence number for the DM-RS by a change in a transmission bandwidth. Further, 30 sequence groups are defined, and in order to reduce intercell interference, different sequence groups are assigned to neighboring cells.
However, when the DM-RS of the LTE is applied to the CoMP terminal “as is”, as will be described below, the DM-RS of the CoMP terminal may not be orthogonal to the DM-RS of the non-CoMP terminal, and thus the reception performance at the time of MU-MIMO may degrade. As illustrated in FIG. 1, a DM-RS transmitted from a one certain CoMP terminal is received at a plurality of cells (base stations). Since the DM-RSs received at the plurality of cells are the same signals transmitted from one terminal, a cell in which the ZC sequence number for the DM-RS of the CoMP terminal is different from the ZC sequence number for the DM-RS of the non-CoMP terminal occurs as in a cell 2 of FIG. 1. Since the ZC sequences having different sequence numbers are not orthogonal to each other, interference (cross correlation) occurs between terminals in this cell, and thus the reception performance at the time of MU-MIMO degrades.
In Non-Patent Literature 1, a Walsh sequence is used to have the DM-RS of the CoMP terminal to be orthogonal to the DM-RS of the Non-CoMP terminal. In the LTE, as illustrated in FIG. 2, two DM-RSs are transmitted through one sub frame; however, each of the two DM-RSs is multiplied by a Walsh sequence ((1,1) or (1,−1)) of which sequence length is 2. A receiver side multiplies the two DM-RSs by the same Walsh sequence as a transmitter side, and performs in-phase addition of the multiplied DM-RSs. If there is no temporal change in a channel between the two DM-RSs, the DM-RSs multiplied by different Walsh sequences by the in-phase addition process (interference components) have phases opposite to each other, and thus interference can be completely removed. Even when neighboring cells have different sequence group numbers (sequence numbers) as in the LTE, if there is no temporal change in a channel, the DM-RSs can be orthogonal to each other by multiplying the DM-RSs by different Walsh sequences.
Further, in Non-Patent Literature 1, it is stated that one sequence group used for generating the DM-RS of the CoMP terminal is selected from sequence groups (30 groups as in the LTE) used in the CoMP set; however, a concrete method of selecting the sequence group is not stated. Further, similarly to the non-CoMP terminal, it is considered to select the sequence group used by a cell (serving cell) that transmits control information to the own terminal.